Turns out it has to do with MobileMe and out of control synchronization.  Here is Damien-007 solution.

I solved this problem a few days ago, after spending close to two hours with Apple Care. All time very well spent since I learned a lot, but more importantly, I thoroughly solved the problem.

Mind you, this was happening to me on my three different computers, 2 iMacs and a Mac Mini, all under 10.6.4
And through solving this problem, I learned that MobilMe also had a sync file on a Windows XP and a Windows 7 machine, neither of which I had booted up, since last spring.

First, the symptoms:
1) your dock changes the icon content before your eyes
2) the dock won’t hold an icon in its place, no matter how you put it there, or what settings or methods you choose to ‘Keep In Dock’
3) you are currently, or have previously used MobileMe, to sync your settings

The cause of these symptoms/problem:
Your MobileMe syncing with the cloud (the Apple servers where they store your sync data and your sync settings) has been corrupted.

Why unchecking Synchronize with MobileMe doesn’t fix it:
Because even on your local machine, the dock is “calling home” to verify it’s settings. When you turn off syncing, it’s inability to call the cloud, continues the problem.

The only guaranteed solution:
You have to Unregister all computer(s) from the MobileMe cloud and start your MobileMe sync fresh (as in brand new settings, NOT Reset settings) on all your computer(s). This applies even if you only sync one computer, and even if you have turned off syncing with MobileMe.

Before beginning, Turn off Synchronize with MobileMe on all computers in your MobileMe cloud.

The steps:
1) Backup everything and take note of all your settings, for which MobileMe has syncing powers over. Specifically, this includes, Bookmarks, iCal, Address Book, Mail Accounts, Mail Rules, Notes, Preferences and Entourage Notes. For Keychains, make sure you scrub that list and delete duplicates. A symptom of corrupt syncing is multiple entries in your key-chain for the same passwords. It’s OK to over delete here since the worst case scenario is you will have to reenter passwords

2) Now you will erase all the data in the MobileMe cloud, 100%.
Go to the Apple (top left corner)/System Preferences/MobileMe/Sync

• click the Advanced button lower right
• click on each computer listed in the pull down window
• to the right of each computer listed, click Stop Syncing Computer
• if you have multiple computers (I had five), each stop will take some time and may produce the beach ball. Be patient and hang in there. Still, all 5 of my computers, only to me about 20 minutes total.
• After the last or ONLY computer has been thoroughly “stopped syncing”,

a NEW pull down window will drop down, listing all the data that lives in the MobileMe cloud

• check mark each file and confirm that you want all that data/entries deleted.

3) Now you need to set up a new dock:

• Delete the dock preferences and database files mentioned by macjack above

“Go to /Users/yourname/Library/Preferences and delete these 2 files /com.apple.dock.plist and com.apple.dock.db.plist Then restart. This will give you the default Dock, so you’ll have to reset yours the way you like.”

Also, make sure your entire computer data is as you expect it. Address Book, iCal etc. Restore from backups if necessary and basically get your Mac to the way you like it.

NOW,
You can set up MobileMe fresh BUT this time do it with the more Advanced method by going back to the Advance Button under
Go to the Apple (top left corner)/System Preferences/MobileMe/Sync

• click the Advanced button lower right

And Register your/each computer: This will automatically start a MobileMe sync.

When asked, tell MobileMe that you want to keep the data on your freshly set up computer and replace the data on Mobile me.

You are done.

Do the above for each computer, for which you use mobileMe with the one variation that you should replace the data on each successive computer, with the sync data on the MobileMe cloud. (after your primary/main computer has thoroughly populated the New MobilMe data in the cloud.